cgen: fix deadlock when returning multiple values in lock (#14014)

vlib/v/gen/c/cgen.v

@@ -3902,6 +3902,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 	tmpvar := g.new_tmp_var()
 	ret_typ := g.typ(g.unwrap_generic(g.fn_decl.return_type))
 	mut use_tmp_var := g.defer_stmts.len > 0 || g.defer_profile_code.len > 0
+		|| g.cur_lock.lockeds.len > 0
 	// handle promoting none/error/function returning 'Option'
 	if fn_return_is_optional {
 		optional_none := node.exprs[0] is ast.None

vlib/v/tests/return_in_lock_test.v

@@ -1,4 +1,3 @@
-// vtest retry: 3
 import time
 
 struct AA {
@@ -6,49 +5,42 @@ mut:
 	b string
 }
 
-const (
-	run_time   = time.millisecond * 200 // must be big enough to ensure threads have started
-	sleep_time = time.millisecond * 250 // some tolerance added
-)
-
 fn test_return_lock() {
 	start := time.now()
 	shared s := AA{'3'}
-	go printer(shared s, start)
-	go fn (shared s AA, start time.Time) {
-		for {
 	reader(shared s)
-			if time.now() - start > run_time {
-				exit(0)
-			}
-		}
-	}(shared s, start)
-	time.sleep(sleep_time)
-	assert false
-}
-
-fn printer(shared s AA, start time.Time) {
-	for {
 	lock s {
-			assert s.b in ['0', '1', '2', '3', '4', '5']
-		}
-		if time.now() - start > run_time {
-			exit(0)
+		assert s.b == '5'
+		s.b = '4'
 	}
+	rlock s {
+		assert s.b == '4'
 	}
 }
 
 fn reader(shared s AA) {
-	mut i := 0
-	for {
-		i++
-		x := i.str()
 	lock s {
-			s.b = x
-			if s.b == '5' {
+		assert s.b == '3'
+		s.b = '5'
 		// this test checks if cgen unlocks the mutex here
 		return
 	}
 }
+
+fn test_multi_return_lock() {
+	shared s := AA{'3'}
+	reti, retb := printer2(shared s)
+	lock s {
+		assert s.b == '3'
+		assert reti == 4
+		assert retb == true
 	}
 }
+
+fn printer2(shared s AA) (int, bool) {
+	rlock s {
+		assert s.b == '3'
+		return 4, true
+	}
+	return 5, false
+}